For downsizing, large capacity and low power consumption of an optical transmitter and receiver, it plays an important role to realize an optical device on silicon. The optical device on silicon, which can use optical waveguide of large refractive index difference, is advantageous in the downsizing in comparison with optical devices on other materials, which facilitates the integration with other electronic circuits, whereby a number of optical transmitters and receivers can be integrated on 1 chip. Of the optical devices, especially the modulator has the characteristics influential on the power consumption and size of the optical transmitter and receiver. Especially, the ring modulator, which is small-sized itself and has a small modulator voltage and a small optical insertion loss, is advantageous in the downsizing and low power consumption.
The followings are examples of related: U.S. Patent Publication No. 2009/0169149 A1; and Japanese Laid-open Patent Publication No. 2008-268276.
However, the ring modulator has the problem of making the modulation efficiency high narrowing the wavelength band.